Harness girth measurement is the widest point of your dog's chest and rib cage area.  Harness sizes are based on the girth measurement and can vary widely between breeds or even individual dogs. It is important to accurately measure your dog's girth for the best possible harness fit.

To measure your dog, stand over your dog and place one end of a measuring tape on the dog's spine just above the widest part of the dog's rib cage.  With your other hand, wrap the tape measure behind the dog's front legs and around the dog's body, making sure to go underneath the widest part of the rib care as shown in the diagram. 

It is important to order a harness that has an appropriate girth measurement for your dog.  If your dog's measurement falls between two different sizes, consider ordering the larger size to allow room for growth.